With the increasing demand for wireless communications, a strong need has developed for better ways to increase the amount of traffic capacity that can be carried in allocated radio spectrum bands. In order to achieve increased capacity, a variety of multiple access approaches have been devised, including the well known approaches of frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA) approaches. In communications systems such as cellular and the emerging personal communication services (PCS), both the older analog (e.g., North American AMPS, or Advanced Mobile Phone Service) and newer TDMA digital services (e.g., the European GSM, or Group Speciale Mobile) rely on some form of frequency division to achieve optimal traffic capacity while maintaining acceptable signal quality.
The conventional approach towards achieving increased capacity in a system using frequency division is to divide a larger service area into smaller "cells," and "reusing" the frequencies within an allocated band among the cells. The amount of reuse permitted, or in other words the minimum distance before a frequency may be repeated in another cell, is typically calculated using a static plan taking into consideration the probable amount of co-channel interference. Co-channel interference is interference arising in one signal's bandwidth from another signal having an overlapping bandwidth. Since the amount of co-channel interference is dependent on the distance between the overlapping-channel broadcasting stations, the chief consideration in static modeling for reuse patterns has been maintaining a sufficient distance between cells reusing the same frequency so that a calculated received signal power in one cell for a signal transmitted in another cell is kept below a predetermined threshold. Improvements have also been made by taking into consideration known structures and seasonal sources (such as foliage) causing readily modeled additional propagation losses.
A shortcoming of all static modeling is that actual propagation patterns within a coverage area will vary, sometimes significantly, from predicted patterns. This variance becomes even more pronounced in smaller cell patterns, such as are typically found in PCS or microcellular systems, where the sources of interference (buildings, trees and the like) are more pronounced in view of the lower antennas and lower transmit power used in smaller rather than larger cellular or trunked radio systems. As a consequence, static modeling alone may leave certain coverage areas too close for actual propagation characteristics, with actual signal quality being degraded beneath desired or even acceptable levels.
An improvement has been proposed for PCS applications in order to safeguard against this loss of signal quality. This improvement, sometimes referred to as a quasi-static automatic frequency assignment (QSAFA) process, uses actual measured signal strengths from surrounding radio ports (base stations), served by a common controller (a RPCU, or radio port controller unit), to determine which of a possible group of frequencies has the lowest signal strength at the measuring radio port. This lowest frequency is selected for use by the radio port, on the assumption that it will have the best performance (i.e., exhibit the least co-channel interference) within the radio port's coverage area. This process is performed for each radio port controlled by the RPCU, and repeated a number of times to insure the best selections for all radio ports.
While QSAFA represents an improvement over static planning, it fails to take into account several other important sources of signal degradation. In particular, neither QSAFA nor static modeling provide any method for measuring and correcting for actual adjacent channel or intermodulation interference. Adjacent channel interference arises from side lobe or spill over energy from one channel overlapping with the transmit band of another channel. Intermodulation interference occurs due to the interaction of multiple carriers in a non-linear device. Approaches such as QSAFA may even exacerbate problems with adjacent or intermodulation interference, since QSAFA always picks the channel with the lowest signal strength, even if it is located immediately adjacent the channel with the strongest signal strength. A need therefore exists for an improved method of allocating frequencies to avoid unacceptable losses in signal quality.